A wide variety of devices for the controlled release of drugs have been proposed. These dosage forms which may be termed "sustained release", "timed release", "prolonged release" or "controlled release" forms are designed to dispense the drug in a controlled reproducible manner. Previous attempts to provide release in a short burst over a period of the order of minutes e.g. those described in U.S. Pat. No. 3,952,741 have not been successful in controlling the delay before which the pulsed release of active material occurs.